Various types of supervisory circuits have been proposed for tractor-trailer combinations, particularly if the tractor vehicle includes brake anti-lock or anti-skid systems, although the trailer does not have such an anti-lock, anti-skid system. It has been proposed--see German Patent Disclosure Document DE-OS No. 22 44 511--to include a supervisory or monitoring terminal or contact in the existing electrical connection terminal between the tractor vehicle and the trailer which, when a trailer which does not have a brake anti-lock or anti-skid system, activates a relay within the tractor which provides a continuous reminder indication signal to the driver of the tractor vehicle that a trailer is connected which has different braking characteristics from those of the tractor vehicle. This arrangement has the disadvantage that the commonly used and standard plug-socket connections between the tractor vehicle and the trailer cannot be used, requiring either adapters or rewiring of the existing plug-socket connectors of both the trailer as well as on the tractor. This is expensive and undesirable and cannot be carried out in practice if standard tractor vehicles are to be connected to randomly selected trailers.
It has also been proposed to provide connecting terminals on the connection plugs themselves which are closed if the plug-socket connection is effected, in other words, if the plug is connected to a socket--or vice versa--upon connection of a trailer, in which the terminal are closed upon mechanical connection of the plug-socket combination. Plug-socket combinations having terminal switch elements have the undesirable characteristic of being unreliable since the terminal elements are exposed--being closed by the mechanical interconnection--and have a tendency to become dirty and make unreliable contact. This problem particularly arises in automotive use where terminals are frequently not maintained with the care that electrical equipment usually deserves. The terminals may be exposed to environmental contamination, dirt, road salt, moisture, and rain if the plug-socket combination is not effected; the reliability of such plug-socket operated connections in automotive applications is questionable and, when most needed, the electrical connections may fail. Additionally, some rewiring of the existing wiring system of the tractor vehicle, at least, is necessary.